Windows are one of the weak points in any home’s heat defences, and a blind is the cheapest thing most people can put over them. The question is simple: do blinds keep heat in, or do they just make a room feel cosier? There is now some decent test data, and it gives a more nuanced answer than most marketing does.
Quick answer: Yes, blinds can reduce heat loss through window glass, but by how much depends on fit. Tests at the University of Salford’s Energy House measured about 13% less heat loss through glazing for a standard roller blind, rising to around a third for sealed-edge designs. Closing blinds at dusk and fitting them snugly matter more than fabric claims.
Do blinds keep heat in?
Blinds really do keep some heat in, because a closed blind adds a layer of still air and fabric between a warm room and cold glass. The effect is real but modest for a loosely fitted blind, and noticeably larger when the blind is sealed at the edges. It works at night, when there is no sun to gain.
Heat leaves a room through a window in three ways. It conducts through the glass and frame, it is carried away by air movement, and it radiates from warm surfaces towards the cold pane. A blind cannot stop any of these completely, but it can slow each one down.
What a blind is not is insulation in the building regulations sense. It does not change the rating of your window, it does not stop draughts coming through a leaky frame, and it only works when it is closed. Think of it as a cheap evening layer, not a replacement for better glazing.
What did the Salford Energy House tests find?
The Salford Energy House tests, commissioned by the British Blind and Shutter Association, found that blinds and shutters reduced heat loss through double-glazed windows by between about 13% and 33%. A standard roller blind sat at the low end. Designs that sealed the sides of the blind, or added a low-emissivity layer, did best.
The testing took place in a full-scale early twentieth-century terraced house built inside an environmental chamber at the University of Salford. According to the BBSA’s published summary of the research, the headline results were:
| Product tested | Reduction in heat loss through the glazing |
|---|---|
| Roller blind with zip side channels and low-e fabric | 33% |
| Blockout honeycomb blind, fitted into the window bead | 32% |
| Internal plantation shutter | 28% |
| Roller blind with fitted side channels | 22% |
| Standard roller blind | 13% |
The pattern is what matters. The same basic roller blind went from 13% to 22% when its sides were closed off, and higher again with a reflective fabric. That tells you edge sealing and air movement matter at least as much as the fabric itself.
How reliable are the figures on blinds and heat loss?
The figures on blinds and heat loss are credible but narrow. They come from a controlled test on one type of modern double glazing, measured at the centre of the glass, and the research was paid for by the blind industry’s trade body. They show a real effect, not a guaranteed saving on your heating bill.
The initial Energy House test report is worth reading for its caveats. It set out that:
- The window was argon-filled double glazing with a low-e coating, tested at 20°C inside and 4.5°C outside.
- Heat flow was measured with a single sensor at the centre of the pane. The report notes that how centre-pane results translate to whole-window heat loss still needs investigating.
- The measurement method carries an uncertainty that the report puts at typically 14% to 28%.
- Single glazing was not tested.
The research is careful, published and consistent with basic physics. But a 13% reduction in heat loss through one part of one window is not a 13% cut in your gas bill. Windows are only one route for heat to escape, alongside walls, roof, floor and draughts, and a blind only works while it is down.
How do blinds reduce heat loss through windows?
Blinds reduce heat loss through windows by trapping a layer of relatively still air against the glass, slowing the convection current that pulls warm room air down the cold pane, and, with reflective backings, bouncing some radiant heat back into the room. The tighter the blind, the better each mechanism works.
The trapped air layer
Still air is a poor conductor of heat. A closed blind creates a pocket of air between the fabric and the glass. If that air stays put, it acts as an extra insulating layer. If it can move freely, most of the benefit is lost.
The convection loop
Air next to cold glass cools, becomes denser and sinks. On a bare window, this creates a steady cold draught down the pane that spills across the floor, even with no leaks in the frame. Behind a loose blind, the same loop runs: warm air enters at the top or sides, cools on the glass and pours out at the bottom. Side gaps and a big gap under the blind keep the loop going. That is why side channels made such a difference in the Salford tests.
Radiant heat and reflective backings
Warm people, walls and furniture radiate heat towards cold glass. A fabric with a metallised or low-emissivity backing reflects some of that back. This is the idea behind most thermal fabrics, and it probably accounts for part of the extra gain seen with the low-e roller blind in the tests.
What matters more than the blind fabric?
Fit and habits matter more than the blind fabric. A blind that sits inside the window recess with small side gaps, closed at dusk every evening, will do more than an expensive thermal fabric hung loosely outside the recess and left open. Radiator position also matters, because a blind can trap heat against the glass.
- Close at dusk, not bedtime. Heat loss through glass is greatest in the evening and overnight. Closing blinds as soon as it gets dark captures most of the benefit.
- Fit inside the recess where possible. A recess fit narrows the side gaps. When we measure, we check whether the recess is deep enough and square enough for a close fit.
- Keep the bottom close to the sill. A blind that stops well short of the sill leaves an easy exit for the cold air loop.
- Watch the radiator. If a radiator sits under the window, a blind that drops in front of it will channel its heat into the space behind the blind and against the glass. A blind that stops at the sill lets that heat into the room instead.
- Open on sunny winter days. Low winter sun through a south-facing window is free heat. Keep blinds up while the sun is on the glass.
Where do blinds make the biggest difference to heat loss?
Blinds make the biggest difference to heat loss on the least efficient windows: single glazing, older double glazing, draughty sash windows and large bays. Basic building physics suggests that adding the same layer of still air cuts more heat from a poor window than from a good one, although the Salford tests only covered modern double glazing.
To be clear, we have not found published UK test data for roller blinds on single glazing that we can link to with confidence. The logic is simple though: the worse the window, the bigger the share of its resistance to heat flow that a blind and its air gap can add.
The types of home where this counts most are common across London, Essex and North Kent:
- Victorian and Edwardian terraces with original single-glazed sash windows, especially in conservation areas where replacing them is restricted. The Energy Saving Trust’s windows advice lists heavy lined curtains and sealed blinds among the non-intrusive options for insulating historic windows.
- 1930s semis with large front bays, where the glazed area is big and older frames can leak.
- Homes with early double glazing whose seals have failed or whose glass lacks a low-e coating.
Bay windows deserve special mention. Each pane of a bay is a separate cold surface, and a single curtain across the front of the bay leaves a large volume of cold air behind it. Individual blinds fitted to each window of the bay keep that cold pocket much smaller. See our guide to roller blinds for bay windows for how that is usually set out.
When do blinds make little difference?
Blinds make little difference when the window is already highly efficient, when the blind hangs loosely with large gaps, when it is left open through the evening, or when the real problem is a draught through the frame. In those cases, the money is often better spent on draught-proofing or simply changing habits.
- Modern triple glazing. The window is already doing most of the work, so the extra gain from a blind is small in real terms.
- Draughts through frames. If air is leaking around a sash or a worn seal, a blind will soften the feel of it but will not stop it. Draught-proofing the frame is the proper fix.
- Loose outside-recess blinds. A blind hung on the wall with big gaps at each side lets the convection loop run freely.
Are thermal blinds worth it?
Thermal blinds are worth it where heat loss through glass is a genuine problem and the blind can be fitted closely, such as single-glazed sashes, bays and older double glazing. They are harder to justify on modern, well-sealed double or triple glazing, where a well-fitted standard blind already captures much of the available benefit.
A thermal roller blind uses a fabric designed to reduce heat transfer, usually with a reflective or coated backing. The Salford results suggest a reflective layer adds something on top of good edge sealing, but not that fabric alone transforms a loose blind. The honest order of priority is:
- Fix obvious draughts around the frame.
- Fit the blind as closely as the window allows.
- Close it at dusk every evening.
- Then consider a thermal fabric for the coldest rooms.
If you want the fabric details, our page on thermal roller blinds explains how they are made and where they suit.
Can blinds that keep heat in cause condensation?
Blinds that keep heat in can increase condensation on the glass. By holding warm room air away from the window, a closed blind makes the pane colder, and any moist air that does reach it condenses more readily. This is most noticeable on single glazing and in bedrooms, where people breathe out moisture all night.
It is not a reason to avoid blinds, but it does mean a routine helps. Open the blind in the morning, wipe the glass if it is wet, and keep trickle vents open. Moisture-resistant fabrics cope better with damp glass than untreated ones. If mould appears on a blind or the window reveal, treat the cause, which is usually too much moisture and too little ventilation, rather than just the blind.
How to get the most warmth from roller blinds
To get the most warmth from roller blinds, fit them inside the recess with minimal side gaps, drop them close to the sill, close them at dusk and open them to let winter sun in. Deal with draughts first and keep radiators on the room side of the blind. These steps cost little and apply to any fabric.
- Check each window for draughts on a windy day and seal frames where needed.
- Measure for a recess fit, allowing only the clearance the mechanism needs.
- Set the drop so the bottom bar sits close to the sill.
- Choose blackout or thermal fabric for the coldest, most exposed rooms.
- Close blinds at dusk, not at bedtime.
- Open them in the morning to let glass dry and sun in.
Our measuring and fitting service covers the recess checks, handle clearances and drop settings that make this work in practice.
Common mistakes with blinds and heat loss
Common mistakes with blinds and heat loss include expecting a blind to fix a draughty frame, hanging blinds over radiators, fitting outside the recess with wide gaps, keeping blinds closed on sunny winter days, and paying for thermal fabric while leaving the blind open all evening. Each one wastes much of the benefit.
- Treating headline percentages as bill savings.
- Choosing an outside fit for looks where a recess fit was possible.
- Covering a bay with one wide blind or curtain rather than one blind per window.
Getting advice on warmer windows locally
Getting advice on warmer windows locally starts with someone looking at the actual windows. We offer a free home measure across London, Essex and North Kent, checking recess depth, gaps, radiators and handles, and bringing fabric samples including thermal and blackout options. You get a written quote with no obligation.
That visit is the right time to ask whether a thermal fabric is worth it for a particular room, or whether a well-fitted standard blind will do. You can check your postcode on our areas we cover page.
Key takeaways
- Lab tests commissioned by the BBSA found a standard roller blind cut heat loss through double glazing by about 13%, and sealed-edge designs by up to about a third.
- Those figures are measured at the centre of the glass and are not the same as bill savings.
- Fit and habit matter most: recess fit, small gaps, close to the sill, closed at dusk.
- Blinds help most on single glazing, older windows and bays, and least on modern triple glazing.
- Deal with draughts first. A blind does not stop air leaking through a frame.
- Warmer rooms can mean colder glass, so open blinds in the morning to control condensation.
Frequently asked questions
How much heat do roller blinds save?
In tests at the University of Salford’s Energy House, a standard roller blind reduced heat loss through double glazing by about 13%. Adding side channels raised that to 22%, and a sealed design with low-e fabric reached 33%. Those figures apply to the glass, not the whole house, so bill savings will be smaller.
Are blinds or curtains better for keeping heat in?
Both work in the same way, by trapping air against the glass, so the better one is whichever fits more closely. A roller blind inside the recess has small gaps. A heavy lined curtain that is floor length and tucked at the sides also performs well. Curtains over radiators waste heat.
Should I close blinds at night in winter?
Yes. Close blinds at dusk rather than bedtime, because heat loss through glass is highest once the sun has gone. Open them again in the morning, especially on sunny days, so the glass can dry and low winter sun can warm the room. The habit matters as much as the blind itself.
Do thermal blinds work on double glazing?
Thermal blinds do work on double glazing, and the Salford tests were carried out on double-glazed windows. The gains are smaller in real terms than on single glazing, because the window already resists heat flow. On modern double glazing, fit and closing at dusk matter more than paying extra for thermal fabric.
Can a blind replace secondary glazing?
No. A blind only works while it is closed and does little for draughts through the frame. Secondary glazing adds a permanent sealed layer of glass. For single-glazed sash windows where replacement is not allowed, a well-fitted blind is a useful addition, but it is not an equivalent to secondary glazing.
Thinking about new blinds?
We measure and fit across London, Essex and North Kent. The visit is free and there is no obligation.
